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Name: Deeptha
Status: student
Age: 16
Location: N/A
Country: N/A
Date: 2001


Question:
Why is mass of neutron greater than mass of proton?


Replies:
Hi, Deeptha !!

Mass of proton : 1,6726 x 10^(-27) kg
Mass of neutron: 1,6749 x 10^(-27) kg
Mass of electron: 0,00091x10^(-27) kg

The mass of a neutron is greater than the mass of a proton because the neutron contains a proton, contains an electron with some subatomic particles.

neutron = proton + electron + subatomic particles

Alcir Grohmann


It is an oversimplification; however, one can consider that a 1 neutron = 1 proton + 1 electron. This accounts for a large part of the difference between the neutron and proton mass. The "real" explanation is much more complicated, but the above explanation is approximately true.

Vince Calder


A neutron does not have a proton and electron within it. The difference in mass is part of what allows a neutron to become a proton, electron, and anti-neutrino. Still, the basic structure of a neutron is three quarks: 1 up, 2 down. Likewise, a proton is three quarks: 2 up, 1 down. An up quark and a down quark are not the same thing. Charge and mass are different. Quarks can be put together as protons, neutrons, and a wide variety of other particles. How they are assembled has a large effect on mass. Due to both quark mass and assembly details, a neutron ends up with more mass than a proton.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


This is a very complicated question with no simple "hand-waving" answer. In energy units (using E = mc^2), the masses are: Proton: 938.272 MeV, neutron: 939.566 MeV, mass difference = 1.293 MeV, electron: 0.511 Mev.

It is tempting to say that a neutron consists of a proton plus an electron; the mass of the electron would make up 40% of the mass difference. This argument is totally invalid. It would be equally valid to say that a proton consists of a neutron plus a positron (a positron has exactly the same mass as an electron, but is positively charged). The validity of using this argument in both directions is strengthened by the fact that neutrons in neutron rich nuclei beta decay into an electron and a neutrino while protons in proton rich nuclei beta decay into a positron and a neutrino. For example a N13 (nitrogen 13) nucleus decays into C13 (carbon 13), a positron, and a neutrino with the release of 2.221 MeV.

The charge of the proton adds some electromagnetic energy to the proton mass, but the magnitude of that effect is not only impossible to calculate, but works in the wrong direction.

Quarks give the best chance to explain the proton-neutron mass difference by "hand-waving". A proton consists (mainly) of two up quarks and one down quark. A neutron consists (mainly) of one up quark and two down quarks. Current estimates are that the up quark has a mass in the range 2-8 Mev and the down quark 5-15 MeV. So replacing one up quark in the proton by a down quark would increase the mass by something between -3 MeV and +13 MeV. Clearly this is not a precise calculation, but it is (mostly) in the right direction and could overcome the electromagnetic contribution and produce the correct answer. There are other known contributions to these masses including interactions with the weak and strong interactions, but this is probably already more than you want to know about this subject!

Best, Dick Plano, Professor of Physics emeritus, Rutgers University



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